The present invention relates to a fixing apparatus and image forming apparatus in, e.g., a color electronic copying machine, printer, or facsimile machine, for fixing an unfixed toner image on a recording medium by fusing.
Some color electronic copying machine has a photosensitive belt for carrying a latent image. A plurality of developing units for selectively supplying toners of respective colors (Y, M, C, and K) are disposed to oppose the photosensitive belt. In this color electronic copying machine, toner images of respective colors formed on the photosensitive belt are transferred onto, e.g., an intermediate transfer body, in an overlaying manner to form a color image. The color image is then transferred at once onto a recording sheet. The recording sheet transferred with the color image is fed to a fixing device. The fixing device fixes the color image on the recording sheet.
The fixing device has a fixing roller serving as a heating roller incorporating a heating source, and a press roller to be urged by the fixing roller. When the recording sheet transferred with the color image passes between the fixing roller and press roller, it is heated and pressed, so the color image is fixed.
In a fixing device of this type, in order to shorten the warm-up time, the heating source such as a heater lamp is arranged only in the fixing roller with which the toner image comes into contact. The press roller does not have a heating source in it. Hence, the press roller need to efficiently receive heat from the fixing roller with the heating source.
The recording sheet passes between the fixing roller and press roller. When image fixing operation is repeated, heat is not sufficiently conducted to the press roller. Then, the surface temperature of the press roller decreases gradually, and defective fixing occurs.
Particularly, in fixing an image such as a color image with a large toner attaching amount, the temperature of the press roller largely influences the fixing properties. If the temperature of the press roller is low, defective fixing tends to occur.
Assume that after copy operation is started, the image of a recording sheet which is fed first between the fixing roller and press roller is to be fixed. In this case, sometimes the press roller does not reach a temperature sufficiently high for fixing. Then, defective fixing occurs.
The present invention has been made in view of the above situations, and has as its object to provide a fixing apparatus and image forming apparatus which can fix an image after the press roller is heated to a sufficiently high temperature.
A fixing apparatus according to the present invention comprises a fixing device, having a heating roller which rotates and a press roller which rotates in tight contact with the heating roller, and adopted to heat and press a medium with an unfixed image by passing the medium between the rollers, thereby fixing the unfixed image, a detection device which detects temperatures of the heating and press rollers, and a control device which variably controls rotating states of the heating and press rollers based on the temperatures detected by the detection device.
A image forming apparatus according to the present invention comprises a rotatable image carrier which carries an electrostatic latent image, an image forming device which forms the electrostatic latent image on the image carrier, a developing device which supplies a developer to the electrostatic latent image, formed by the image forming device, by rotation of a developing roller, thereby forming a developer image, a transfer device which transfers the developer image developed by the developing device onto a medium, a fixing device, having a heating roller which rotates and a press roller which rotates in tight contact with the heating roller, and adopted to heat and press a medium with an unfixed image by passing the medium between the rollers, thereby fixing the unfixed image, a detection device which detects temperatures of the heating and press rollers, and a control device which variably controls rotating states of the heating and press rollers based on the temperatures detected by the detection device.
An image forming apparatus according to the present invention is an image forming apparatus comprising a rotatable image carrier which carries an electrostatic latent image, an image forming device which forms the electrostatic latent image on the image carrier, a plurality of developing devices which selectively supply developers of different colors to the electrostatic latent image, formed by the image forming device, by rotation of a developing roller, thereby forming developer images of respective colors, a transfer device which transfers the developer images of the respective colors developed by the plurality of developing devices onto a medium in an overlaid manner, a fixing device, having a heating roller which rotates and a press roller which rotates in tight contact with the heating roller, and adopted to heat and press a medium with an unfixed image by passing the medium between the rollers, thereby fixing the unfixed image, a detection device which detects temperatures of the heating and press rollers, and a control device which variably controls rotating states of the heating and press rollers based on the temperatures detected by the detection device.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.